<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Undefined 和 Null</title>
</head>

<body>
    <script>
        var person;
        var car = "Volvo";
        document.write(person + "<br>");
        document.write(car + "<br>");
        var car = null;
        document.write(car + "<br>");
    </script>
</body>

</html>